Maybe this flew under the radar (or I misinterpreting things), but given your system volume configuration isn’t the same as the default_level (set up by mycroft.config/VolumeSkill itself) there is no adjustment at the initialisation of the skill. As i understand the code _setvolume is only event driven. Am I wrong?
And vol_before_mute would keep it that way, never really setting default_level unless you fire up the event (eg set it yourself).
In my case the state got stored at 75 vol. No matter the default (VolumeSkill; default_level=6) nor the custom entry (mycroft.conf; default_level=10) makes any difference.